Project 3 SUMMARY/ABSTRACT Angiogenesis is known to play a critical role in cancer growth and metastasis. Among the many potential targets, vascular endothelial growth factor (VEGF) has been well recognized to play an important role in angiogenesis, and drugs targeting this pathway have been used against ovarian and other cancers. Clinical use of anti-VEGF therapy, however, has yielded only modest improvement in progression-free or overall survival of patients with ovarian cancer, likely due to adaptive changes in the tumor microenvironment. There remains an unmet need to develop methods to enhance efficacy of anti-VEGF therapy and block growth- promoting adaptive changes. The mechanisms of adaptive resistance to anti-VEGF treatment are largely unknown. Understanding the adaptive resistance to anti-VEGF treatment has the potential to significantly enhance the efficacy of anti-VEGF therapy in ovarian cancer patients. Our preliminary findings suggest that tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) are substantially increased in the anti-VEGF therapy-resistant tumors and TAM depletion (with CSF1R inhibitor) can improve the effectiveness of anti-VEGF therapy; however, the mechanisms by which this occurs are not well understood. In this proposal, we will explore the mechanisms by which macrophages contribute to adaptive resistance to anti-VEGF treatment and test the efficacy of dual targeting of VEGF and TAMs/MDSCs. Our central hypothesis is that targeting TAMs in the microenvironment will reverse the immunophenotypical alterations induced by bevacizumab and improve clinical efficacy. We will conduct a novel, induction, randomized supplementation clinical study to assess the impact of adding a CSF1R inhibitor to identify and overcome these effects as measured by objective response and event-free survival. The proposed work is highly translational and has the potential to significantly enhance the efficacy of anti-VEGF therapy in ovarian cancer patients.